A wide variety of processes for the manufacture of cross-linked thermoplastic articles are practised commercially. As an example of such processes, there may be mentioned the preparation of a thermoplastic polymeric material, e.g., polyethylene composition containing, inter alia, a chemical cross-linking agent such as a peroxide, which undergoes rapid decomposition with the concomitant generation of free radicals, by which cross-linking is initiated, at and above a certain temperature i.e. the decomposition temperature of the peroxide. The choice of the particular peroxide is based on a number of factors especially the processing temperature of the thermoplastic. The composition containing peroxide can be molded using conventional molding methods such as blow, transfer or injection molding if the article is to be a hollow article. The composition is then held at an elevated temperature in the mold for a time sufficient to effect cross-linking. Many crosslinked materials require irradiation for cross-linking and the instant invention is addressed to products requiring materials that must be irradiated. Specifically, this invention applies to products that require the use of polymer compositions of exceptional chemical stability and/or physical strength in which compositions cannot be cross-linked chemically. In addition, the instant invention applies to situations in which greater demands are made by the configurations, tolerances and cycle times of the molded article, and it is therefore necessary that the composition be molded, injection molded, transfer molded or the like.
U.K. Pat. No. 1,336,869, states "Because thermoplastic polymers lose their characteristic thermoplasticity when cross-linked, they cannot be molded by conventional procedures after being cross-linked and if articles of cross-linked polymer are required, the article must first be formed from the uncross-linked polymer and the cross-linking treatment then carried out on the article." If the shape of the injection molded article is complex, e.g., it includes a branch, or a lip or flange projecting into its interior, substantial difficulty is experienced in removing the uncross-linked article from an interior male former or core. Thus molding of complex articles presents particular difficulties in that blow and transfer molding and conventional injection molding all have fundamental shortcomings. The present invention is directed to solving the problems involved in precisely forming a cross-linked article of complex shape from a thermoplastic which is not amenable to chemical cross-linking.